You might want to take a look at Michael Tsai's iPhone text editor's comparison. It's not specifically for iPad, but summarizes (and he keeps it more or less updated). You have links to all the apps he mentions and a nice formatted table with the "features" each editor has or lacks.

He specifically added iPad support in the linked version of the entry:

I’ve added rows for tracking iPad

And of course, at the time of writing this…

WriteUp, still my preferred editor, syncs faster with Dropbox than before and now supports .taskpaper files.

_{Bear in mind that Michael updates the entry every now and then. I've linked to version six, but for the sake of this AskExchange entry, you should check if there aren't new ones. Use a google search like this. If you don't know what google is, you can try this other link. ;)}